1. Field of the Invention
This invention relates to a microelectromechanical system (hereinafter abbreviated as MEMS) microphone, and more particularly, to a resistant-type MEMS microphone.
2. Description of the Prior Art
A MEMS device is a microscopic device that is fabricated using the same methods (e.g., the deposition of material layers and the selective removal of material layers) that are used to fabricate conventional analog and digital CMOS circuits. And the microscopic devices include both the electronic and mechanical function which is operated based on, for instance, electromagnetic, electrostrictive, thermoelectric, piezoelectric, or piezoresistive effects. Therefore, MEMS structures are often applied to microelectronics such as accelerometer, gyroscope, mirror, and acoustic sensor, etc.
MEMS acoustic sensors (also referred to as MEMS microphones) are apparatus for converting a voice to an electric signal. The MEMS microphones include piezoelectric-type MEMS microphone, piezoresistive-type MEMS microphone, and capacitive-type MEMS microphone. In the conventional piezoelectric-type MEMS microphone, piezoelectric material is used to form a diaphragm. In operation, changes in air pressure (e.g., sound waves) cause the diaphragm to vibrate which, in turn, causes charges proportional to the vibration. And thus, sound waves are converted into electrical signals. In the conventional piezoresistive-type MEMS microphone, piezoresistive material is used to form the diaphragm. In operation, sound waves cause the diaphragm to vibrate which, in turn, causes resistance changes proportional to the vibration. And thus, sound waves are converted into electrical signals. The capacitive-type MEMS microphone uses a principle of a capacitor where two electrodes face each other, where one electrode is fixed onto a substrate and the other electrode is suspended in the air such that a vibrating plate reacts to an external acoustic pressure so as to be moved. When distance between the two electrodes changes due to the sound waves, the capacitance changes and thus sound waves are converted into electrical signals.
However, the piezoelectric-type and piezoresistive-type MEMS microphones respectively require specific piezoelectric and piezoresistive materials for forming the diaphragm. Furthermore, the piezoelectric-type and piezoresistive-type MEMS microphones suffer low sensitivity and high noise. On the other hand, the distance between the two electrodes of the capacitive-type MEMS microphone must be made to an exact specification. And the fixed and flexible electrodes must experience low stress so that they would not bend and alter the inter-electrode distance between them. It is found that the capacitive-type MEMS microphone may fail when the two electrode bend or contact each other. Accordingly, the exact specification of the electrodes and the distance between the two electrodes contributes to the design complexity and affects the manufacturing yield of the capacitive-type MEMS microphone. And thus the capacitive-type MEMS microphone suffers high process cost and high process complexity.
Therefore, a different type of MEMS microphone is still in need.